Spirit is not feeling like itself lately. The Mars rover that has defied expectations by far surpassing its expected 90-day lifespan on the planet did not report into NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., over the weekend and did not record its activities from Sunday.

On Sunday, Spirit did not heed commands to drive to its next destination on the Martian surface, and stayed put instead. Then on Tuesday, Spirit failed to properly locate the sun with its camera when told to do so by mission controllers.

It appears that the aging rover is returning back to normal, however. “Right now, Spirit is under normal sequence control, reporting good health and responsive to commands from the ground,” NASA’s rover mission chief John Callas noted.

But Daniel Cressey suggests that that Spirit’s problems may simply be the result of teenage rebellion. “Maybe the rover is just entering that difficult teenage phase. Maybe it’s growing tired of being told what to do all the time. ‘Sweep up this dirt, go over there, find me a glass of water.’ Can you blame it if it wants a lie in on a Sunday? Give the poor thing some freedom.”

Spirit and Opportunity were expected to explore the Red Planet for only 90 days before becoming inoperable. Spirit arrived first on, Jan. 3, 2004, and Opportunity landed 21 days later on the other side of the planet. But five years later, the rovers are still chugging. The scientists don’t know how much longer the rovers are going to survive—they could break down tomorrow or years from now.

“The American taxpayer was told three months for each rover was the prime mission plan,” said Ed Weiler, NASA’s associate administrator for science missions, reports LiveScience. “The twins have worked almost 20 times that long. That’s an extraordinary return of investment in these challenging budgetary times.”

The rovers have been searching for any signs of past or present life on the seemingly barren planet. Spirit has come across evidence of salts in the soil, a possible sign of seawater. Opportunity has uncovered small pellet-sized deposits, called “blueberries” by NASA, “that are rich in hematite, a mineral that forms only in watery environments,” according to Time magazine. And in 2007, Spirit discovered white silica, another sign of water.

The two Mars rovers have covered more than 13 miles of the Martian surface in their years on the job, and have sent back to Earth about 36 gigabytes of data, including 250,000 images. NASA is particularly thankful that the rovers have been so the prolific, as the agency had to delay the Mars Science Laboratory (MSL) mission that was scheduled for last month, Agence France-Presse reports. The 2.3 billion-dollar laboratory is now slated for a 2011 launch.

Yearly operating costs for the rovers run the government about $20 million each year. Spirit is currently exploring a plateau called Home Plate, which has a history of volcanic material, LiveScience writes. Opportunity, meanwhile, is now moving toward Endeavour Crater; the trip is only seven miles, but the deliberate and carefully moving rover will not reach its destination for two years. By the time it gets there, it may have expired.

“This last winter was a squeaker for Spirit,” John Callas, NASA’s rover project manager at Jet Propulsion Laboratory, said. “We just made it through.”

Summers on the Red Planet have hovered around 68 degrees Fahrenheit, but during the winter, the machines have had to survive temperatures of –148 degrees, according to AFP.

One of Spirit’s wheels is now badly damaged, and the rover now has to drive backward, “dragging its broken wheel in the soil as it moves along,” according to Time. Spirit’s twin is having problems of its own: “Opportunity’s robotic arm—which carries many of its exploration tools, including its rock drill—has what amounts to an arthritic elbow. This makes it impossible for the arm to retract fully and requires the rover to toddle along with a sort of perpetual salute.”

In September 2007, NASA announced that it hopes to place a man on Mars by 2047. “We are planning many missions. Our long-term game-plan is to put man on Mars by 2037, so that by 2057, when the International Aeronautical Congress (IAC) holds its centenary, we should be celebrating the 20th year of putting man on the red planet,” NASA administrator Michael Griffin said, according to Monsters & Critics.

But he also noted that NASA is “looking beyond moon and Mars into the inter-planetary system,” and planning “to build a space civilisation for tomorrow and beyond that.”

In November, NASA lost touch with the Phoenix lander. The lander, which arrived earlier in 2008, also exceeded expectations for its duration as it functioned at a high-latitude in much more inclement conditions than faced by the rovers, the BBC reports.